1. Field of the Invention
The invention relates generally to the field of surface flow. More particularly, the invention relates to total vector maps of surface flow. Specifically, a preferred implementation of the invention relates to using data from a single HF (high frequency) radar to construct total vector maps of surface flow.
2. Discussion of the Related Art
Coastal radars operating from MF through VHF are finding wide application for mapping the important surface flows that transport floating bodies, vessels, and pollutants. The Doppler shifts of echoes scattered from short-period surface gravity waves allow the measurement of one component of the two-dimensional surface velocity vector. For backscatter geometries where a transmitter and receiver are colocated, the measured component points toward or away from the radar, and is referred to as the radial velocity. When transmitter and receiver are separated, the radar geometry is known as bistatic. In this case, the component measured from the Doppler lies along hyperbolas confocal about the transmitter and receiver locations.
Conventional practice requires two or more such radars to observe a point on the ocean in order to construct a total velocity vector from each of the one-dimensional components measured by a single radar. Furthermore, the angle between the velocity components must avoid the near-parallel condition where the two are essentially the same; the best angle between them is a right angle. Finally, the point on the ocean at which the two-dimensional vector is desired must be viewable by both radars. That is, it cannot extend beyond the maximum range of either, nor can it be shadowed by land or coastal features. These conditions typically eliminate most of the radar data taken by one or the other radars. In the past, the only way to solve this problem has been to install additional radar units, driving up the cost and inconvenience of the observation system while demanding greater radio spectral bandwidth resources to accommodate more radiated signals.
Methods have been proposed and tested to construct total velocity vector current maps from single-site data. One used an equation of continuity approximation, based on the incompressibility of water. More recently, other approximations have been tried that involve time and space averaging of the radial vector data, sacrificing resolution in both of these variables and accuracy in the vector itself. Neither of these methods has proven sufficiently reliable for widespread acceptance and use.
U.S. Pat. No. 4,996,533, was issued in 1991 on a technique that purported to produce total vector maps from a single radar, obtaining the transverse velocity component with a spaced-antenna correlation method. Barrick (1990) proved that this method works only when flow direction is known with respect to the radar beam. Since this direction is unknown, that method provided nothing beyond the radial maps previously available, and its widespread commercialization never occurred. Many have observed that rigorous construction of total vectors from single-radar data only, in the manner of the above references, suffers from the inability of radial data fields to represent unambiguously circulation structures containing vorticity or curving flows, an essential feature of current circulation.
Recently Lipphardt et al. (2000) recognized that these methods could be applied to the total-vector two-dimensional current maps being produced by coastal HF radars on the open ocean. They began referring to the technique as xe2x80x9cnormal mode analysisxe2x80x9d (NMA), and applied it to CODAR SeaSonde (CODAR Ocean Sensors; Los Altos, Calif.) measurements near Monterey Bay, Calif. As most users of HF surface-current data, they attacked the problem as follows: they employed the measurements only where total-vector data overlap from two or more radar sites. Thus, they eliminated data from perhaps 70% of the measurement area where only one radar could observe the sea. Their problem was compounded because most of their boundary was xe2x80x9copenxe2x80x9d, meaning they had no shore to constrain the flow for normal mode computation. Therefore they had to rely on estimates of currents on the open, artificial boundary that were not independently measured.
As mentioned earlier, the problem with conventional methods that construct total vectors from single site radial data is that they have been hampered by the inability to resolve vorticity-containing rotational flow features.
Another problem with the current technology is the inability to obtain meaningful surface current maps in areas where inadequate or no data are available to form a direct two-dimensional vector from two scalar radar components at that point.
There is a need for the following embodiments. Of course, the invention is not limited to these embodiments.
An aspect of the invention is a method to synthesize total surface current vector maps. The method includes extracting a scalar data set from a radar signal from a radar. Radial velocity components are calculated from the radar signal. The radial velocity components are fitted in a linear least-squares manner to a set of scalar eigenfunctions and constants called normal modes. This set of normal modes are used to create a two-dimensional vector field or a current map.
These, and other, embodiments of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings.